1. Field of the Invention
The present invention relates to an image sensing apparatus and imaging system.
2. Description of the Related Art
In an image sensing apparatus disclosed in Japanese Patent Laid-Open No. 11-150255, a capacitance C, amplifier 2, switch SW, capacitance Csh, and amplifier 5 are connected, in the order named, to a signal line SIG, which is connected to pixels on each column in a pixel array, as shown in FIG. 28. In this image sensing apparatus, a reset switch 1 resets the capacitance C in accordance with a reset signal “rc”, as shown in FIG. 29. Then, a thin-film transistor T is turned on, in accordance with a transfer signal “gx”, to transfer the signal of a photoelectric conversion element S to the capacitance C, and the signal is stored in the capacitance C. After that, when the switch SW is turned on in response to a pulse “smpl”, the amplifier 2 amplifies the signal stored in the capacitance C. The amplified signal is transferred to the capacitance Csh in a sample-and-hold circuit 3, and stored in it. After the switch SW is turned off, the signal stored in the capacitance Csh is transferred to an A/D converter 7 when an analog multiplexer 4 selects terminal 4 in accordance with pulses ad0 to ad8. The A/D converter 7 A/D-converts the received signal and outputs the converted signal to Dout. This arrangement can prevent the adverse effect of fluctuations of an analog voltage output from the amplifier 2 on a signal stored in the capacitance Csh while the switch SW is OFF. According to Japanese Patent Laid-Open No. 11-150255, the apparatus can easily improve the S/N ratio.
In an image sensing apparatus disclosed in Japanese Patent Laid-Open No. 2003-51989, a noise signal and optical signal are read out from a pixel to an output amplifier at different timings. The output amplifier performs CDS processing to output the difference between the optical and noise signals. According to Japanese Patent Laid-Open No. 2003-51989, the image sensing apparatus can output an image signal free from fixed pattern noise.
Image sensing apparatuses have been required to perform the readout operation of a signal from a pixel to an output amplifier at higher speed, and to output an image signal generated in accordance with a signal read out from a pixel with lower noise contained in it. It is generally difficult to meet both of these two requirements.